Development of ultrasensitive micro- and nano-electromechanical systems (MEMS/NEMS) has resulted in ultra-high detection sensitivity, offering sub nanometer scale displacement detection, zeptogram level mass sensing, single bio-molecular sensing, and atomic resolution imaging. Micro and nanocantilevers, as MEMS/NEMS transducers, have been used extensively for these sensing applications. Optical transduction of cantilever motion is almost exclusively used to achieve high deflection sensitivity (in the femtometer range), but it suffers from high power requirement, challenges with miniaturization and array based operation. Femtometer scale displacement detection using nanocantilevers operating at several hundred MHz has been demonstrated, but is limited by its challenging fabrication and integration schemes, coupled with complicacies of impedance matching for high frequency signal transmission. Silicon (Si) based piezoresistive microcantilevers have been developed which are easily integrated for array based operation, but have low sensitivity offering displacement resolution in the range of nanometers.
Instead of a simple piezoresistor, embedding a transistor at the base of the microcantilever (henceforth to be called a “piezotransistive” microcantilever) to transduce its deflection is an attractive way to dramatically improve its sensitivity by orders of magnitude, since the gate can be utilized to control the charge carrier density and the mobility of the carriers in the channel.
Recently, metal oxide semiconductor field effect transistor (MOSFET) integrated Si cantilevers have been proposed with the goal of achieving very high deflection sensitivity while avoiding the challenges associated with the aforementioned techniques. Although these microcantilevers showed high sensitivity in the nm range for step deflections, since its high sensitivity supposedly originated from trapping effects in the MOSFET, it is difficult to reproduce these sensors, or operate them at high frequencies. Indeed, Si based piezotransistive microcantilevers are theoretically incapable of exhibiting direct sensitivity enhancement through gate control, since the piezoresistive effects in Si originate from the variation in carrier mobility due to strain related splitting of the conduction band minima energy levels. On the other hand, piezotransistive cantilevers made of piezoelectric materials can directly utilize the charge density variation caused by the deflection induced strain to exhibit high sensitivity with very high repeatability.
Due to strong piezoelectric properties of AlN and GaN, AlGaN/GaN heterojunction, provides a unique avenue to translate the static piezoelectric charge generated at the interface due to applied strain into a change in resistance of the two dimensional electron gas (2DEG) formed at the interface, since the generated piezoelectric charge can proportionately modulate the density of the 2DEG. In addition to changing the carrier density, the applied strain can also change the carrier mobility by changing their effective mass. The utility of AlGaN/GaN heterojunction based piezoresistor (for step bending and dynamic deflection measurements) and piezotransistor (for static deflection measurements) has been demonstrated, however, the effect of gate in enhancing displacement sensitivity down to femtometer range in high frequency dynamic deflection mode, with subsequent applications in unique analyte detection, has never been realized.